A rail vehicle axle bears the weight of the vehicle. The rail vehicle axle is further subjected to a horizontal force caused by contact between the wheel and the rail every time the vehicle passes a curved rail (curve passing). In other words, the rail vehicle axle is repeatedly subjected to rotating bending stress for every one rotation of the wheel. And the amplitude of the bending stress increases at the time of curve passing.
Such a rail vehicle axle is required of a high fatigue limit. Particularly, an axle needs to be provided with a fitting part with a wheel, a gear, or a bearing because of its structural requirements. It is known that the fitting part is subjected to damages due to fretting fatigue. Moreover, in the non-fitting part, there is a risk of the occurrence of flaws and pits due to flying stones and corrosion in addition to damages due to ordinary fatigue, and decrease of fatigue limit resulting therefrom.
Japanese Patent Application Publication No. 06-33219 (Patent Literature 1), Japanese Patent Application Publication No. 10-8204 (Patent Literature 2), Japanese Patent Application Publication No. 10-8202 (Patent Literature 3), Japanese Patent Application Publication No. 11-279696 (Patent Literature 4), Japanese Patent Application Publication No. 2001-206002 (Patent Literature 5), and Japanese Patent Application Publication No. 2000-73140 (Patent Literature 6) propose a rail vehicle axle having an excellent fatigue limit.
Patent Literature 1 discloses as follows. The rail vehicle axle of this literature is subjected to ion nitriding treatment. As a result, a fitting part of the axle to be fitted into a wheel has a surface compound layer made up of Fe4N(γ) phase and having a thickness of 10 to 20 μm, and immediately below thereof, a diffusion layer having a maximum hardness of not less than 280 in Hv. Patent Literature 1 states that this results in an axle having a high fatigue limit.
Patent Literatures 2 and 3 disclose as follows. The rail vehicle axles disclosed in these literatures contain, in mass %, C: 0.3 to 0.48%, Si: 0.05 to 1%, Mn: 0.5 to 2%, Cr: 0.5 to 1.5%, Mo: 0.15 to 0.3%, and Ni: 0 to 2.4%. In a surface portion of this axle, onto which the wheel is fitted, there is an effective hardened layer, in which Vickers hardness is not less than 400, and which has a depth in a range of 1 to 4.5 mm, and in the inner part thereof, there is a martensite or bainite region. Patent Literatures 2 and 3 state that the above described rail vehicle axles have a high fatigue limit.
Patent Literature 4 discloses as follows. The rail vehicle axle disclosed in this literature contains, in mass %, C: 0.3 to 0.48%, Si: 0.05 to 1%, Mn: 0.5 to 2%, Cr: 0.5 to 1.5%, Mo: 0.15 to 0.3%, and Ni: 0 to 2.4%. The fitting part of this axle has a hardened layer having a Vickers hardness of not less than 400, and in the inner part thereof, a region of tempered martensite or bainite. In this axle, the depth of the hardened layer is not less than 5.0 mm, and not more than 10% of the diameter of the fitting part. Patent Literature 4 states that the above described rail vehicle axle has a high fretting fatigue limit.
Patent Literature 5 discloses as follows. The rail vehicle axle disclosed in this literature contains, in mass %, C: 0.3 to 0.48%, Si: 0.05 to 1.0%, Mn: 0.5 to 2.0%, Cr: 0.5 to 1.5%, Mo: 0.15 to 0.30%, and Ni: 0 to 2.4%. The above described axle has 0.2% proof stress of 700 to 1200 MPa. Further, surface portions of both the fitting part and the fillet part of the above described axle have a hardened layer formed by pressing or shotpeening. Literature 5 states that the above described rail vehicle axle has a high fretting fatigue limit.
Patent Literature 6 discloses as follows. The rail vehicle axle disclosed in this literature contains, in mass %, C: 0.3 to 0.48%, Si: 0.05 to 1%, Mn: 0.5 to 2%, Cr: 0 to 1.5%, Mo: 0 to 0.3%, and Ni: 0 to 2.4%. A fitting end part and a peripheral region thereof of the axle have a hardened layer having a Vickers hardness of not less than 400. The ratio (K/D) of the thickness (K) of the hardened layer to the diameter (D) of the fitting part is 0.005 to 0.05. The upper portion of the hardened layer contains 0.02 to 2% of B. Literature 6 states that the above described rail vehicle axle has an excellent fatigue limit.